Single crystal photovoltaic devices, especially silicon photovoltaic devices, have been utilized for some time as sources of electrical power because they are inherently non-polluting, silent and consume no expendable natural resources in their operation. However, the utility of such devices is limited by problems associated with the manufacture thereof. More particularly, single crystal materials (1) are difficult to produce in sizes substantially larger than several inches in diameter, (2) are thicker and heavier than their thin film counterparts; and (3) are expensive and time consuming to fabricate.
Recently, considerable efforts have been made to develop processes for depositing amorphous semiconductor films, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n type devices substantially equivalent to those produced by their crystalline counterparts. It is to be noted that the term "amorphous" as used herein, includes all materials or alloys which have long range disorder, although they may have short or intermediate range order or even contain, at times, crystalline inclusions. Also, as used herein, the term "microcrystalline" is defined as a unique class of said amorphous materials characterized by a volume fraction of crystalline inclusions, said volume fraction of inclusions being greater than a threshold value at which the onset of substantial changes in certain key parameters such as electrical conductivity, band gap and absorption constant occur.
It is now possible to prepare by glow discharge or other vapor deposition techniques, thin film amorphous silicon or germanium based alloys in large areas, said alloys possessing acceptable concentrations of localized states in the energy gaps thereof and high quality electronic properties. Suitable techniques are fully described in U.S. Pat. No. 4,226,898, entitled "Amorphous Semiconductor Equivalent to Crystalline Semiconductors," of Stanford R. Ovshinsky and Arun Madan which issued Oct. 7, 1980 and in U.S. Pat. No. 4,217,374, under the same title, which issued on Aug. 12, 1980, to Stanford R. Ovshinky and Masatsugu Izu, and in U.S. Pat. No. 4,504,518 of Stanford R. Ovshinsky, David D. Allred, Lee Walter and Stephen J. Hudgens entitled "Method of Making Amorphous Semiconductor Alloys and Devices Using Microwave Energy," which issued on Mar. 12, 1985, and in U.S. Pat. No. 4,517,223 under the same title which issued on May 14, 1985 to Stanford R. Ovshinsky, David D. Allred, Lee Walter and Steven J. Hudgens, the disclosures of which are incorporated herein by reference. As disclosed in these patents, it is believed that fluorine introduced into the body of amorphous semiconductor alloy material operates to substantially reduce the density of the localized states therein and facilitates the addition of other alloying materials.
It is of obvious commercial importance to be able to mass produce photovoltaic devices such as solar cells. However, with crystalline cells, mass production was limited to batch processing techniques by the inherent growth requirements of the crystals. Unlike crystalline silicon, amorphous silicon and germanium alloys can be deposited in multiple layers over large area substrates to form semiconductor devices such as solar cells in a high volume, continuous processing system. Such continuous processing systems are disclosed in the following U.S. Pat. Nos. 4,400,409, for A Method of Making P-Doped Silicon Films And Devices Made Therefrom; 4,410,588, for Continuous Amorphous Solar Cell Deposition And Isolation System And Method; 4,542,711, for Continuous Systems For Depositing Amorphous Semiconductor Material; 4,492,181 for Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells; and 4,485,125 for Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells. As disclosed in these patents, the disclosures of which are incorporated herein by reference, a substrate may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific semiconductor material. For example, in making a solar cell of n-i-p type configuration, the first chamber is dedicated for depositing a n-type amorphous silicon alloy, the second chamber is dedicated for depositing an intrinsic amorphous silicon alloy, and the third chamber is dedicated for depositing a p-type amorphous silicon alloy.
The layers of thin film semiconductor alloy material thus deposited in the vacuum envelope of the deposition apparatus may be utilized to form a photovoltaic device including one or more p-i-n cells, one or more n-i-p cells, a Schottky barrier, as well as photodiodes, phototransistors, other photosensors, memory arrays, display devices or the like. Additionally, by making multiple passes through the succession of deposition chambers, or by providing an additional array of deposition chambers, multiple stacked photovoltaic cells or other semiconductor devices of various configurations may be fabricated.
Following the deposition of the layers of semiconductor alloy material upon the substrate, a further deposition process must be performed either in a separate environment or as a part of a continuous process. In this step a thin, transparent or semi-transparent layer of electrically conductive, light transmissive material comprised of, for example, an alloy of indium, tin and oxygen (ITO) or indium and oxygen (10), is deposited atop the layers forming the body of semiconductor alloy material. In the case of photovoltaic cells or photosensors, this transparent, conductive layer forms one of the electrodes thereof. It is the process of and apparatus for depositing such a thin, conductive, transmissive layer in superposed electrical communication with a body of semiconductor material, to which the present invention is primarily directed.
There are a wide variety of such transparent conductive materials having utility in the fabrication of semiconductor devices, such as photovoltaic cells. Generally speaking, materials such as degenerate semiconductors, wide band gap semiconductors, thin metallic films and cermets may be utilized to form the transparent conductive layer. Among some of the specific materials which may be utilized are indium oxide, tin oxide, indium tin oxide, cadmium oxide, cadmium stannate, zinc oxide, silicon carbide and various combinations thereof. It is to be understood that other transparent conductive materials may also be utilized in the practice of the instant invention. While the description herein will primarily be concerned with the deposition of thin, transparent, electrically conductive materials upon subjacent layers of semiconductor alloy material as a step in the fabrication of photovoltaic devices, it should be understood that such transparent conductive films also have utility in other electronic devices such as liquid crystal displays, photosensors, light emitting diodes, photochromic and electrochromic devices and the like. Furthermore, it should be clearly understood that the instant invention may be employed in any instance where the fabrication of a semiconductor device necessitates the plasma coating of a layer of material atop a semiconductor body which is prone to damage during the coating process.
There are a number of techniques utilized to deposit layers of transparent conductive material atop semiconductor bodies. Vacuum evaporation is one such technique. A typical vacuum evaporation process is carried out in a chamber maintained within a pressure regime substantially below atmospheric, typically in the range of 10.sup.-3 to 10.sup.-6 Torr. A charge of the material to be evaporated is placed into a crucible and heated by resistance, induction or electron beam bombardment to produce a vapor thereof, which vapor condenses upon the semiconductor body which is supported in close proximity to the crucible.
There are a number of variations of this typical vacuum evaporation process. For example, in a reactive evaporation process one component of the material being deposited is introduced as a gaseous reagent in the deposition chamber and the remaining components are charged into the crucible. A vapor phase reaction takes place with the product of that reaction depositing upon the substrate (such as the body of semiconductor alloy material). For example, one or more metals may be evaporated in the presence of oxygen to produce a transparent, conductive oxide film. In further variations of this process, electron beams, activated plasmas or other such energetic input may be used to facilitate the vapor phase reaction of the depositing species.
While vacuum evaporation techniques do have the advantage of being relatively simple, they are not always well suited for the commercial, high volume preparation of semiconductor devices. First of all, vacuum evaporation processes require relatively low pressures thus necessitating lengthy and complex pumping procedures. Furthermore, scale-up of a vacuum evaporation process from a research apparatus to a commercial scale, continuous production apparatus is relatively difficult because of the high degree of dependence of the process parameters upon the geometry of the deposition system. Furthermore, it has been found that in many applications evaporated coatings manifest poor adhesion with the underlying layers upon which they are deposited.
In contrast to vacuum evaporation, plasma processes such as sputtering, glow discharge, plasma activated evaporation and the like are fast, easy to control and scale-up, provide highly adherent coatings and consequently may be advantageously employed in the fabrication of layers of transparent conductive material. In a typical sputtering process, a d.c. or radio frequency signal is employed to generate ions from a working gas maintained at a pressure of typically 10.sup.-3 torr. Such ions are strongly attracted to, and consequently bombard, an electrically biased target (also referred to as a cathode), thereby ejecting particles of the target material, which particles deposit onto the exposed surface of a substrate maintained in close proximity thereto. In the preparation of a layer of indium tin oxide for example, the face of the target or cathode is a body of solid indium tin oxide material. A working gas, typically argon, is ionized and attracted to the target. The energetic impingement of the argon ions ejects small particles of the indium tin oxide material from the target, which particles condense upon the exposed surface of a semiconductor layer disposed upon a substrate. There are numerous variations of this basic sputtering process, such as magnetron sputtering in which a magnetic field associated with the target is utilized to enhance the efficiency of the process. In another notable variation, referred to as reactive sputtering, a post ejection reaction of the particles from the surface of the cathode with the gaseous atmosphere in the sputtering apparatus occurs so as to provide a new material therefrom, which material then condenses upon the substrate. For example, a target of an indium-tin alloy may be sputtered in an atmosphere of oxygen to provide a coating of indium tin oxide.
In a typical glow discharge deposition process, a gaseous reagent is introduced into a low pressure environment and subjected to electromagnetic energy so as to create an activated plasma therefrom. In this plasma, the gaseous reagent mixture reacts to form species which subsequently deposit on a substrate maintained within the low pressure environment. In an activated evaporation, vapor of a coating material is subjected to an energetic input so as to create a plasma therefrom for facilitating creation and maintenance of desirable coating species. It should be noted that there are a wide variety of plasma coating processes known and available to those skilled in the art and the instant invention, as will be detailed hereinbelow, may be employed in conjunction with any such process wherein it is desired to protect a body of semiconductor alloy material from damage due to the energetic impact thereupon of depositing species.
As mentioned hereinabove, plasma processes are attractive for use in the deposition of thin layers of transparent, electrically conductive material in the manufacture of electronic devices because such processes are easy to control, can achieve high deposition rates and may be readily scaled-up for large volume production. However, the use of such sputter coating or other plasma processes has often been found to result in damage to semiconductor layers upon which the coating material is being deposited. Such damage decreases the efficiency or other operational parameters of the electronic devices and in some instances can render them completely inoperative. Damage during plasma coating results from the energetic impingement of ionic or other energetic species upon the semiconductor layers of the substrate. Such bombardment can produce mechanical damage to the semiconductor layers, which damage is manifest as broken chemical bonds, vacancies in the matrix of the semiconductor material, and/or over-or-under-coordinated valencies in atoms from which the semiconductor alloy material is formed. In other instances, the ions impinging upon the semiconductor layer are reactive species themselves and as such partially denature the semiconductor layer; for example, oxygen or nitrogen ions can produce oxide or nitride species in the host matrix of the semiconductor alloy material thereby dramatically changing its optical, chemical and electrical properties. In other instances, reactive ions can create an interfacial layer between the depositing layer and the semiconductor layer, thereby limiting good electrical contact of the deposited layer with the semiconductor body.
In general, mechanical damage to or the chemical reaction occasioned by ions or other energetic species depositing upon the body of semiconductor alloy material produces defect states in the semiconductor body, which defect states may adversely affect the operation of the device being fabricated therefrom. For example, it has been found that when a layer of indium tin oxide is sputter coated onto a body of amorphous silicon or amorphous germanium alloy layers for forming a photovoltaic device, such layers will be damaged due to the sputtering process. It has been found that in some instances this damage may be mitigated by employing a very slow rate sputtering process; however, a slow process is impractical in commercial scale production. Accordingly, there is a need for a process for the high deposition rate sputtering of thin film materials onto semiconductor layers, which process does not occasion damage to those layers.
There is a great need for high deposition rate plasma coating processes which do not damage underlying semiconductor layers. The instant invention meets this need by providing protective means in a coating system, for preventing damage to the underlying layers of semiconductor alloy material, thereby allowing for the hiqh deposition rate coating thereof. Such a process may be readily adapted for the high deposition rate plasma coating of a wide variety of materials, as a step in the fabrication of photovoltaic devices, photosensors, imaging devices, display devices and other opto-electronic devices. The instant invention thus makes possible large scale, high yield, commercial production of a wide variety of semiconductor devices.
These and other advantages of the instant invention will be apparent from the brief description, the drawings and the detailed description thereof which follow.